Lurasidone [chemical name: (3aR,4S,7R,7aS)-2-{(1R,2R)-2-[4-(1,2-benzisothiazol-3-yl)piperazin-1-ylmethyl]cyclo-hexylmethyl}hexahydro-4,7-methano-2H-isoindole-1,3-dione] of the following formula is a compound having a pharmacological activity as an antipsychotic agent, which is characteristic of a high affinity for dopamine D2, serotonin 5-HT1A, 5-HT2A, 5-HT7, and noradrenaline α2C receptors, and characteristic of minimal to no affinity for histamine H1 and muscarinic M1 receptors. Lurasidone possesses antipsychotic effects, antidepressant- and anxiolytic-like effects, and pharmacological profiles with potentially-reduced liability for extrapyramidal and CNS depressant side effects, which is expected to be used for the treatment of schizophrenia and bipolar disorder (Patent Reference 1, Non-patent Reference 1).

International Statistical Classification of Diseases and Related Health Problems (also known as “ICD-10”) includes “Mental and behavioural disorders” (F00-F99) as a category in Chapter V which further includes “Pervasive developmental disorders” (F84) as a small-classified disease in a subcategory “Disorders of psychological development” (F80-F89). According to ICD-10, the pervasive developmental disorders are furthermore classified into 8 diseases, i.e., childhood autism (“autism” in ICD-10), atypical autism, Rett's syndrome (Rett syndrome), other childhood disintegrative disorder, overactive disorder associated with mental retardation (or intellectual disability) and stereotyped movements, Asperger's syndrome (Asperger syndrome), other pervasive developmental disorders, and unspecified pervasive developmental disorder. Amongst them, autism is a typical disease thereof, and it is sometimes used as a generic term thereof. Autism is characterized by the development of impairments in social adaptive function, impairments in communication, and perseveration of specific behavior/interest. In addition, patients of genetic and/or congenital diseases such as fragile X syndrome, congenital rubella syndrome, cytomegalic inclusion body disease, and phenylketonuria can develop an autism-like symptom.
The prevalence rate of autism is thought to be about 1%, and it is reported that the men's prevalence is three to nine times higher than the women's, i.e., it is known that there is some difference on the incidence rate of the disease between men and women (Non-patent Reference 2). Patients of autism generally have core symptoms such as impairments in social adaptive function and impairments in communication, and peripheral symptoms such as abnormal repetitive behavior, hyperactivity, impulsivity, and irritability. It is known that autism is developed during childhood, but its core symptoms (impairments in social adaptive function and impairments in communication) can still continue even in adulthood. Thus, it is indicated that patients of autism have social disadvantages.
The current method for treating autism includes various programs such as TEACCH (Treatment and Education of Autistic and related Communication handicapped CHildren), and Social Skills Training (SST), which are based on educational treatment. The purpose of these programs is to promote the communication in patients of autism and relieve a difficulty in their social lives, but there is no curative treatment. The drug therapy for autism is only a symptomatic therapy with a drug useful for peripheral symptoms thereof. For example, in order to treat irritability which is one of transient peripheral symptoms in autism, some drugs such as aripiprazole and risperidone are used as a symptomatic therapy (Non-patent References 3 and 4). However, there is no useful drug for impairments in social adaptive function or impairments in communication which are core symptoms in autism.